Wireless devices for communication such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, Base Transceiver Station (BTS), or AP (Access Point), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro NodeB, home NodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for terminals. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies. A Radio Network Controller (or RNC) is a governing element in the UMTS radio access network and is responsible for controlling the Node Bs that are connected to it. The RNC carries out radio resource management, some of the mobility management functions and is the point where encryption is done before user data is sent to and from the mobile. The RNC connects to a Circuit Switched Core Network through Media Gateway (MGW) and to a SGSN (Serving GPRS Support Node) in a Packet Switched Core Network.
Soft HandOver (SHO), also referred to as macro diversity, and fast closed-loop power control are essential features of WCDMA and Enhanced UL (EUL). Soft handover is a feature in which a UE is simultaneously connected to two or more cells. This means that data is received simultaneously in multiple cells, which provides diversity gains. Typically it is distinguished between soft and softer handover, where softer handover refers to the case when the SHO cells belong to the same site, e.g. NodeB, whereas in soft handover the cells can belong to different sites, e.g. NodeBs. FIG. 1 illustrates SHO operation for a traditional High Speed Packet Access (HSPA) deployment scenario with two nodes, a macro node of a serving cell and a macro node of a non-serving cell, having a similar transmit power level. The main difference between Macro, Micro, Pico nodes is the output power, and the more output power the larger coverage area. A Macro has typically 20 W or 40 W, i.e. 43 dBm or 46 dBm or a power in-between or similar. Ideally, a UE moving from the serving cell towards the non-serving cell will enter a SHO region at point A. This may be referred to as Event 1a. At point B also referred to as Event 1d, a serving cell change will occur, i.e. a non-serving cell becomes a serving cell and vice versa. At point C also referred to as Event 1b, the UE will leave the SHO region. It is the Radio Network Controller (RNC) that is in control of reconfigurations, which implies rather long delays for e.g. performing a cell change. During SHO, the UE is power-controlled by the best uplink cell. Since the nodes have roughly the same transmit power in FIG. 1, the optimal DL and UL cell borders will coincide, i.e. the path loss from the UE to the two nodes will be equal at point B. Hence, in an ideal setting and from a static, such as long-term fading, point of view, the serving cell will always correspond to the best uplink. However, in practice, due to imperfections such as e.g. reconfiguration delays, and fast fading, the UE may be power controlled by the non-serving cell during SHO. In such case there may be problems to receive essential control channel information in the serving cell due to the weaker link between the serving cell and UE. For example, High Speed Dedicated Physical Control Channel (HS-DPCCH) and scheduling information need to be received in the serving cell. Possible remedies include increasing gain factors by means of Radio Resource Control (RRC) signaling, utilize repetition or rely on HARQ. The power of each UL physical channel is set relative the HS-DPCCH power by means of its gain factor, i.e. P_channel_X=gain_factor_channel_x^2×P_DPCCH. Further details related to gain factors is described in 3GPP 25.213 version 11.3.0, and 25.214 version 11.3.0. Note though that possible imbalances between UL and DL are mainly caused by fast fading in a traditional deployment, whereas for other scenarios, e.g. heterogeneous networks, other factors make the imbalance more pronounced.
During the 3GPP RAN#56 plenary meeting, a Study Item (SI) was initiated on UMTS Heterogeneous Networks, ‘RAN Plenary (RP)-111642, “Work Item (WI): MIMO with 64QAM for HSUPA”, Nokia Siemens Networks. Deployment of Low-Power Nodes (LPNs) in the heterogeneous networks is seen as a powerful tool to meet the ever-increasing demand for mobile broadband services. A LPN may correspond, for example, to a Remote Radio Unit (RRU), a pico base station or a micro base station, allowing expanding the network capacity in a cost-efficient way. RRU is a radio transceiver capable of handling multiple UEs simultaneously. Typically, several RRUs are connected to and controlled by a single central controller. A network comprising traditional macro nodes such as macro NodeBs, and LPNs is referred to as a heterogeneous network. Two examples of use-cases for heterogeneous network deployment that may be envisioned are coverage holes and capacity enhancement for localized traffic hotspots.
Since the LPNs and macro NodeBs in a heterogeneous network have different transmit powers, the UL and DL cell borders will not necessarily coincide. An example of this is when a UE has a smaller path loss to the LPN, while the strongest received power is from the macro NodeB. Path loss is the reduction in power density of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of a link budget of a telecommunication system.
In such a scenario, the UL is better served by the LPN while the DL is provided by the serving macro node. This is shown in FIG. 2 which illustrates SHO operation for HSPA in a heterogeneous deployment. The region between the equal path loss border and equal downlink received power, e.g. Common Pilot Channel (CPICH) receive power, border is referred to as imbalance region. In this imbalance region, some fundamental problems may be encountered. For example, a UE in position A would have the macro node as the serving cell, but be power controlled towards the LPN. A UE in SHO is power controlled by several cells, e.g. all cells in the active set. All cells compares the received UE DPCCH power with a SIR target, where the SIR target is set by the RNC. If the received power is larger than the target then the cell orders the UE to reduce its power and vice versa if the received power is less than the target. The UE combines the power control commands from the multiple cells into a single command. It is the strongest link that dictates the power control, i.e. it is enough that one cell orders the UE to decrease its power, while all other cells orders UP for the UE to decrease the power. Due to the UL-DL imbalance the UL towards the serving macro node will be very weak, which means that important control information, such as scheduling information or HS-DPCCH, may not be reliably decoded in the serving cell. Furthermore, a UE in position B will have the macro node as the serving cell, and also be power controlled towards the macro. Due to the UL-DL imbalance, the UE will cause excessive interference in the LPN node. Furthermore, in this scenario the benefits of macro node offloading towards the LPN cannot fully be utilized. One way of improving these problems is to utilize available RNC based cell selection offset parameters. For example, by tuning Cell Individual Offset (CIO) parameter the handover border can be shifted towards the optimal UL border. Handover is based on DL CPICH power. If the UE discovers that the CPICH power from a non-serving cell is stronger than the CPICH from the current serving cell, then the UE will inform the network about this, also referred to as event 1c. The CIO may be used to offset this handover measurement. The CPICH power from one cell, e.g. the LPN, may appear to be stronger or weaker than it actually is, thereby triggering handover earlier or later. There are network parameters that may be used to tune the measurements used in UE event procedures based on the scenario, e.g. Macro-to-Macro, Macro-to-LPN, LPN-to-LPN, etc. The effect of these adjustments is illustrated in FIG. 3. FIG. 3 illustrates SHO operation for HSPA in a heterogeneous deployment with range extension.
These adjustments are beneficial from a system performance point of view, but some difficulties remain:
Scenario 1—A UE in position A may experience a poor DL from the non-serving LPN. This may complicate a reliable detection of UL related DL channels by the UE, e.g. E-HICH and F-DPCH from the LPN.
Scenario 2—A UE in position B has the macro node as serving cell but is in general power controlled towards the LPN. Hence, the uplink signal towards the serving cell may be weak and thereby complicate a reliable reception of control channel information at the serving cell.
Scenario 3—A UE in position C is served by the LPN. However, its DL may be poor and thereby complicate a reliable reception of control information, such as High Speed-Shared Control Channel (HS-SCCH) and Enhanced Absolute Grant Channel (E-AGCH).
Scenario 4—A UE in position D may experience a poor UL towards the non-serving macro cell and thereby complicate the uplink reception at the macro cell.
To maximize the potential gains provided by range extension, the problems associated with the different scenarios above need to be solved. This will allow not only to optimize the system performance, but also to improve the link quality for UEs experiencing severe degradation in UL or DL.